1. Field of the Invention
The present invention relates to a sheet feeding apparatus and an image forming apparatus and more specifically relates to a configuration to separate and feed sheets to an image forming portion one by one.
2. Description of the Related Art
Recently, an image forming apparatus such as a printer, a copying machine, or a facsimile has a sheet feeding apparatus to separate and feed sheets to an image forming portion one by one, and such a sheet feeding apparatus has a sheet separating and feeding portion to separate and feed the sheets one by one. Such a sheet feeding apparatus adopts, so to speak, a retard separating method, having a feed roller which rotates in the sheet conveying direction, a retard roller which presses the feed roller, and a torque limiter which is connected to the retard roller, as disclosed in Japanese Patent Laid-Open No. 5-338837.
A pressing force of the retard roller against the feed roller and a torque value of the torque limiter are set so that the retard roller may rotate along with the feed roller in a case where a nip portion of the feed roller and the retard roller has no sheet or only one sheet. Also, the pressing force of the retard roller against the feed roller and the torque value of the torque limiter are set so that only a sheet contacting the feed roller may be conveyed without the retard roller rotating along with the feed roller in a case where plural sheets enter into the nip portion of the feed roller and the retard roller.
FIG. 9 illustrates a configuration of a conventional sheet feeding apparatus which has a sheet separating and feeding portion in such a retard separating method. In FIG. 9, a cassette 200 stores (supports) sheets S, a side wall 203 of the cassette 200 is on the downstream side in the sheet feeding direction, and a medium plate 201 is provided in the cassette 200 to be movable in the up and down directions. The sheets S are loaded on the medium plate 201.
A pickup roller 129 feeds an uppermost sheet Sa stored in the cassette 200. A feed roller 130 is made of, for example, EPDM, and a retard roller 131 presses the feed roller 130 by a not-illustrated spring and is made of, for example, urethane.
The retard roller 131 presses the feed roller 130 to form a separating nip portion N between the retard roller 131 and the feed roller 130 in a state where a surface portion of the retard roller 131 contacting the feed roller 130 is compressed. The uppermost sheet Sa fed by the pickup roller 129 is fed to the separating nip portion N formed by the feed roller 130 and the retard roller 131. A conveyance path 303 is formed by a conveying guide 302 provided from the separating nip portion N to a pair of conveying rollers 305a and 305b. 
In a case of sheet feeding in the conventional sheet feeding apparatus configured as above, the pickup roller 129 first rotates and conveys the uppermost sheet Sa stored in the cassette 200 to the separating nip portion N. In a case where only one sheet Sa is fed to the separating nip portion N, driving of the retard roller 131 is interrupted by the effect of a torque limiter, and the retard roller 131 rotates to accompany the sheet Sa, as illustrated in FIG. 10A. Thereby, the sheet Sa passes the conveyance path 303 and is conveyed.
On the other hand, in a case where plural sheets are conveyed to the separating nip portion N, the retard roller 131 does not rotate along with the feed roller 130 but rotates in a reverse direction of the sheet conveying direction of the feed roller 130 by the effect of the torque limiter. The reverse rotation of the retard roller 131 causes only one sheet Sa contacting the feed roller 130 to be conveyed and causes the other sheets to be returned to the upstream side in the sheet feeding direction by the retard roller 131 as illustrated in FIG. 10B. This can prevent multiple feed of sheets.
Meanwhile, in the conventional sheet feeding apparatus and an image forming apparatus having the same, the feed roller 130 and the retard roller 131 are abraded as use time goes by. When they are abraded much, a frictional force between a sheet S and the retard roller 131 and a torque of the torque limiter+a rotational resistance of the retard roller 131 may have relationship shown in Equation (1) shown below.Frictional force between sheet and retard roller<(torque of torque limiter+rotational resistance of retard roller)/roller radius  (1)
In this case, when a sheet is conveyed to the separating nip portion N, the retard roller 131 stops without being rotated to accompany the sheet conveyed by the feed roller 130. That is, an accompanying rotating failure of the retard roller 131 occurs. Nevertheless, even in a case where the retard roller 131 is in a state of the accompanying rotating failure, sheet separation and conveyance are performed if relationship in Equation (2) shown below is established when one sheet is conveyed to the separating nip portion N.Frictional force between feed roller and sheet>Frictional force between sheet and retard roller  (2)
Also, sheet separation and conveyance are performed if relationship in Equation (3) shown below is established even when two or more sheets are conveyed to the separating nip portion N.Frictional force between sheets<Frictional force between sheet and retard roller  (3)
Further, even in a case where the retard roller 131 is in a state of the accompanying rotating failure, a frictional force between the retard roller 131 and the feed roller 130 is larger than the frictional force between the sheet and the retard roller 131. Accordingly, relationship in Equation (4) shown below is established. Before the sheet enters the separating nip portion N, the retard roller 131 rotates, and thus the sheet enters the separating nip portion N and is conveyed.Frictional force between retard roller and feed roller>(torque of torque limiter+rotational resistance of retard roller)/roller radius  (4)
However, the retard roller 131 is not abraded uniformly, but abrasion of an abraded part is advanced locally when the abrasion occurs. Also, when use of the image forming apparatus is stopped, the retard roller 131 stops in a state where the specific part at which the abrasion is advanced presses the feed roller 130 more frequently, in which case a dent is generated at the specific part. When this state is maintained for a long time, the dent at the specific part becomes larger as illustrated in FIGS. 11A and 11B.
When the dent becomes larger in this manner, the rotational resistance of the retard roller 131 is increased. When the feed roller 130 rotates in this state, the surface of the retard roller is abraded. When this phenomenon is repeated, a large dent caused by the abrasion is generated only at the specific part of the retard roller 131. As a result, decrease of the left-hand side of Equation (1) and increase of the right-hand side of Equation (1) are promoted only at the specific part of the retard roller 131, and a part at which relationship in Equation (5) shown below is established is generated at an early stage.Frictional force between retard roller and feed roller<(torque of torque limiter+rotational resistance of retard roller)/roller radius  (5)
When the part having such relationship is generated, the retard roller 131 is in a stop state before a sheet enters the separating nip portion N as illustrated in FIG. 11A. This interferes with entering of the sheet S into the separating nip portion N, and a jam of the sheet S occurs before the separating nip portion N as illustrated in FIG. 11B. In this manner, when the dent is generated at the retard roller, the jam of the sheet S occurs, and the sheet cannot be separated and fed.
The present invention provides a sheet feeding apparatus and an image forming apparatus enabling to prevent generation of a dent at a retard roller.